Stabilized feed-back condenser microphone



Jan. 10, 1950 w. R. HARRY 2,493,819

STABILIZED FEEDBACK CONDENSER MICROPHONE Filed Nov. 25, 1947v 2 Sheets-Sheet 1 F/G/ Fi I' 1 /5 4 2? 22 26 Si; `w- 24 Si i 2 I Ll; 25 is /9 i E n 2o la /6 a *1 Ii' Il l?. EAMP ,/44 Aaa/o FEED EQUAL/ZEE OUTPUT ALE/IME I 1 l ifi. gsi/g l 43 46 15 1c 23 f 47 /NVENTOR W @HARRY Jan. 10, 19500 w. R, HARRY I 2,493,819

STABILIZED FEEDBACK CONDENSER MICROPHONE Filed Nov. 25, 1947 2 Sheets-Sheet 2 e2 rom/#.42

8 50ml F/as ,6

PHASE /N DEGREES LOSS IN 08 ,oa ail o lo ao' looo |0000 00000 FREQUENCY IN CYCLES PER .SECUND BV l A TTOR/VEY Patented Jan. l0, 1950 UNITED STATES PATENT OFFICE STABILIZED FEED-BACK CONDENSER MICROPHONE William R. Harry, Milwaukee, Wis., assignor to Bell Telephone Laboratories,

Incorporated,

This invention relates to electro-acoustic transducers and more particularly to transducers of the type, such as disclosed in Patent 2,387,845 granted October 30, 1945, to William R. Harry, including a microphone having a directional response characteristic and associated with an amplifier to constitute a stabilized negative feedback system therewith.

General objects of this invention are to improve the performance and to facilitate the construction of electro-acoustic transducers and especially of such devices including an electro-static or condenser type microphone.

More specifically objects of this invention are to obtain a high signal-to-noise ratio for electroacoustic transducers, to reduce distortion, due to wind noise, in such transducers, to lessen the susceptibility thereof to noise due to electrical leakage across insulators in the microphone and associated circuit elements, to relieve the stringency of mechanical design requirements in electro-static or condenser microphones having a diaphragm interposed between a pair of electrodes and to facilitate the realization of uniform response over a wide range of frequencies.

In one illustrative embodiment of this invention, an electro-acoustic transducer comprises a directional, specifically cardioidal, microphone including a diaphragm vibratile between a main and an auxiliary electrode, an amplifier energized in accordance with variations in the capacitance between the diaphragm and main electrode as the diaphragm vibrates in accordance with sound waves effective thereon, and a feedback circuit coupled to the auxiliary electrode and energized proportionately to the amplifier output for effecting control of the diaphragm motion.

In accordance with one feature of this invention, the condenser microphone is radio frequency polarized such that the input to the amplifier is a radio frequency carrieror signal -amplitude modulated in accordance with vibrations of the diaphragm. Because of the low effective resistance in the input circuit to the amplifier, re-

' sulting from the low reactance of the microphone at the carrier and side-band frequencies, a high signal-to-noise ratio is obtained. Also, susceptibility of the circuit to noise due to leakage across insulators therein is lessened. In one specific form, the diaphragm and main electrode constitute one capacitative arm of a bridge circuit, the adjacent arm of which is dened by a condenser substantially .balancing the microphone capacitance. The radio frequency polarizing voltage is applied across these two capacitative arms as by way of a center-tapped secondary `winding of a transformer, the two halves Aof this winding constituting the other two arms of the bridge.

In accordance with -another feature of this invention, the output of the amplifier is detected to regain the audio frequency envelope? and a portion of the detected output is applied between the diaphragm and the auxiliary electrode to apply feedback control effective upon the diaphragm down to zero frequency. Such feedback control at zero frequency tends to center the diaphragm whereby necessity for exactv initial equality of the diaphragm to electrode spacings is reduced. Further, feedback at zero and low frequencies reduces distortion due to wind noise.

In accordance with a further feature of this invention, the feedback circuit includes a network for compensating, over a range of low frequencies in the band to be translated by the transducer, for the rising response characteristic of the microphone due to its directional character, whereby the response is equalized over this range, and an additional network is provided for` equallzing the response at higher frequencies. This enables equalization over the entire band to be translated without degradation of the signal-to-noise ratio and simplifies the construction of the Output audio amplifier energized from the detector by facilitating the realization of sufficient gain `at this amplifier to enable use of local feedback for the reduction of noise and distortion thereat.

The invention and the above-noted and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is a side elevational view, mainly in section, of a condenser microphone which. may be included in an electro-acoustic transducer illustrative of this invention; y

Fig. 2 is an enlarged sectional view of a portion of the microphone illustrated in Fig. 1,showing details of the diaphragm and electrode' mountings; I

Fig. 3 is a circuit diagram, principally in block form, of a transducer illustrative of one embodiment of this invention;

Fig. 4 is a diagram illustrating the bridge circuit included in the transducer shown in Fig. 4;

Fig. 5 is another diagram illustrating the low frequency equalizer included in the transducer; and

Fig. 6 is a graph showing the transmission the I:mounting :ring 24.

characteristics of the equalizer illustrated inI Fie. 5.

Referring now to the drawing, the microphone illustrated in Figs. 1 and 2 is of the electrostatic or condenser type and comprises a metallic housing including a cylindrical body fzportion -Iil provided with .aiplurality 1of apertures II ,covered with acoustic resistance material I2, such as acoustic silk. The housing includes also an perforate Vrear closure I3 and 4an annular front flange I4. Amxed against the yharige I4 by 1a locking ring I5 threaded to the housing portion IIIY is `a thin, disc diaphragm 4.4i-, der example .ofY

sheet aluminum substantially 10111102?! inch thick,

gaskets I'I, which also may be of aluminum, ":hef ing provided on opposite sides .ofi the diaphragm adjacent its periphery.

Opposite the inner face of the diaphragm-.i6

diaphragm tofback electrode spacing. This Snacing :is fixed byfa metallic :annular spacer 2 I ofpreassigned thickness interposed between :the ring ,la and '-diaphragm. A .pressure or clamping 22 is threaded tothe :housing 'portion 'If Yand 'bears against-the ring =IL9 :and by `rotation thereof 'the diaphragm is tensioned to .the Adesired jdegree and resonance Vfreomency .Xed, for example at about 2,000 ley/cles per second;

A second unitary .assembly is-mounted opposite the other face -offthe diaphragm and comprises a perforated front -or auxiliary electrodeV T23, 'a mounting ring 24 and an insulating, e. fg. polystyrenefannul'us 25 moulded tothe .electrode 23 and "ring` 24. Thefaees of the electrode .273 and rng 24 toward the diaphragm are lapped to vbe accurately jcoplanarthespaeing of the Ielectrode '23 from the diaphragm `is' vfixed by -an annular metallic spacer '26 disposed between the ldiaphragm and the ring 2-4.

Vthreaded 'to the 'ange i4 and bearing against an annular spring 28 which in turn bea-rs against Electrical connection to the electrodes I8 and 23 may be established by way of coaxial lines 'the outer conductors 429 of which are connected -to the housing Yand have at their outer endsv insulating caps 30 carrying terminal-s 31 for the inner conductors, One ofthe inner conductors V32 is connected to the front or auxiliaryeiectrode 23 and passes through-an insulating sleeve '3 3 in ,respectye outer conductor, The other inner cm'iductor'is .coupled to the main or back electrode VI8 by a tie wire 34 andipasses through the respective outercon- The stiffness of the chamber Ilriovided to the -riearof the diaphragm Vby the housing is corisrelai'edv with the mass and resistance provided by the acoustic material i2 :and the external con- 'ngfuration of thehensing is made such, both lin Y Y m m v The front electrode assenfiblyjis held in place by a locking 'ring/21 Ways known in the art, that the microphonephas Y .a'vpresmihedhirectional response pattern. ge. gs

sound Waves incident normal to the outer or front face of the diaphragm.

The microphone above-described is utilized as a portion of a stabilized negative feedback system in which the microphone is radio frequency polarized. Specically, .asillustrated in Fig. 3, the'diaphragm It and mainfor back electrode I3 are connected by a balancing condenser 36 in V:series with a center-tapped transformer winding 3l, the other winding 38 of the transformer being :energized from a'radio frequency oscillator 39. The oscillator frequency is not critical and may be, -for example, .between 0.5 megacycle and 5 megacycles. -A afrequency of one megacycle has been 'found satisfactory, particularly from the standpoint of transformer design. As illustrated in Fig. 4, the `diaphragm It, electrode I8, condenser St and transformer winding 31 constitute a bridge, the two halves of the winding 31 constituting twoarms of the.V bridge, .the capacitance Gil, .being that between the diaphragm I6 and back ormain `electrode I'S ,-.constituting the third arm and :the condenser 13E constituting the fourth ,armand balancing the capacitance 40. Advantageously, V the .bridge Yis slightly unbalanced in the-direction such that .an increase in the capacitance 40 results in a further unbalance.

The `voltage .e1 across element .4I represents the carrier or .radio frequency voltage due to the bridge unbalance plus that of Ysideband fre- `culencies when sound .waves are incident upon the diaphragm, and may/be considered asthe amplitude .of .the-.envelope .modulation .at audio frequencies. For 10.0 per cent coupling between .the two .halvesbf transformer winding 31, it is apparent from Fig. 4 that the output voltage e2 is `equal to audio frequency envelope. A portion ofthe de-V :tector outputis supplied to the front or auxiliary electrode 23 over `a feedback circuit 44, thereby to produce a `controlling force acting upon the diaphragm. f

The remainder of -thc audio frequency output @of the detectorV 43 vis supplied -to .an audio vfre- -miency amplifier 45 by way of :an equalizer 46 the function of which will appear presently.V Advantageouslm the amplifier 45 is provided with feedback 41 to attain high level undistorted audio output. Y f

VAs is kno-wn, for example as pointed out in the -ililliam R. Harry --patent.heretofore identified, in :a microphone having-a directional, e. g. card-ioida-lfresponse, the lriet acoustic force effective upon the diaphragm varies with frequency be- .cause of the dependence upon frequency of the relative phase of the acoustic :forces acting upon opposite faces of Vthe diaphragm. As pointed out further in that patent, Vthis variation innet force'may be compensated for by application of feedback o f a prescribed character, to the auxili-ary electrode. Y Y

In the transducer thus far described herein and message .nee-effect of faded. the serial 0btainedjat the output 'ofthe' detector-win' increase with frequency, speciflcally will increase at the rate of approximately 6 decibels per octave. This increase and consequent non-uniformity in the frequency response characteristic theoretically might be accounted for by providing a single equalizer which would introduce a loss increasing at the same rate with frequency. Practically, however, this solution is not satisfactory for the reason, among others, that such an equalizer would yield a voltage so low that the noise generated in the first stage of the audio amplifier Would be greater than the noise component at this point in the system due to the microphone, with the result that the signal-to-noise ratio of the transducer would be degraded.

In accordance with one feature of this invention, the desired compensation for the rising microphone characteristic is attained without degradation of the signal-to-noise ratio of the transducer and with the realization, expeditiously and economically, of adequate gain in the audio frequenci7 amplifier to enable adequate feedback at this amplifier for the reduction of noise and distortion in this portion of the system.

Specifically, the feedback circuit 44 is constructed to equalize the response over a range of lower frequencies in the band to be translated and the equalizer 46 is constructed to compensate for the rising microphone characteristic at frequencies above this range. For example, in a typical transducer intended to translate faithfully al1 frequencies within the band from 30 to 15,000 cycles, the feedback circuit may be constructed to effect equalization in the response between 30 and 1,000 cycles and the requalizer' 46 may be constructed to provide uniformity in the band from 1,000 to 15,000 cycles.

A suitable form of the feedback cincuit 44 is illustrated in Fig. 5 and its transmission characteristics are illustrated in Fig. 6. It comprises a series resistor 48, the parallel resistor-condenser combination 49, 50, the series resistor condenser combination 5I, 52 and the shunt condenser 53, all connected as shown between the detector 43 and the diaphragm i6 and front or auxiliary electrode 23. In a typical network having the phase and loss characteristics illustrated in Fig. 6, the parameters of the impedances of Fig. 5 are as follows:

It will be seen from Fig. 6 that the loss introduced by the network illustrated in Fig. 5 increases substantially linearly over the frequency range from 30 to 1,000 cyclesatgsubstantially 6 decibels per octave and is substanti-ally constant above this range. Thus, the network compensates for the rising microphone characteristic over this frequency range. The departure from linearity of the loss characteristic adjacent the lower extreme frequency, i. e. 30 cycles, is small and, if desired, may be corrected by design of the amplifier and feedback system.

This microphone, radio frequency supplier, detector and feedback in combination constitute an electromechanical system of the configuration of the a circuit described in the article Stabilized feedback amplifiers, Iby H. S. Black in the Bell System Technical Journal, January 'andere 1934, page f1, the bridge i6, i8, 36 and 31, amplifier 42 and detector 43 constituting the p. portion of the circuit and the feedback circuit 44 and electrode 23 constituting Vthe portion of the circuit. In general, the components are correlated in accordance with known principles, set forth in the Black article, so that they constitute -a stabilized feedback system. That is to say, the phase and gain conditions in the system are made such that the a gain is substantially greater than unity over the requisite frequency range, bearing in mind that the phase conditions in the radio frequency circuit elements affect the phase of the audio frequency modulation envelope. In a specific circuit, a a gain of approximately 45 decibels at the diaphragm resonance frequency (about 2,000 cycles per second), sloping at a rate of 10 to 12 decibels per octave at higher frequencies to a value of unity at approximately kilocycles, and, at frequencies below the diaphragm resonance, rst falling to a minimum of 15 decibels at approximately 15 cycles and then rising again to 45 decibels at zero frequency, together with a phase which passes degrees at approximately 200 kilocycles may be employed.

The equalizer 46 may be a resistance-condenser network having a loss characteristic which rises at substantially 6 decibels per octave above the frequency, e. g. 1,000 cycles, at which the loss characteristic of the equalizer in the feedback path 44 levels off, and, thus, compensates, above this frequency, for the rising characteristic of the microphone.

Certain characteristics of the transducer are to be noted particularly. Inasmuch as the microphone impedance is low at the high carrier frequency employed, the bridge circuit operates at a low impedance level. Hence, the effective input resistance for the transducer is small and additionally is substantially constant for the range of audio frequencies translated. Inasmuch as the amplifier 42 operates at radio frequencies, the low frequency fluctuation noise of the first stage tube thereof is of no moment. Consequently, a high signal-to-noise ratio is obtained.

Feedback control is obtained over the ,circuit 44 down to zero frequency. Hence, the feedback tends to center the diaphragm I6 between the electrodes I8 and 23 whereby stringency of equality of the two diaphragm-to-electrode spacings is reduced. Also, because of feedback at zero and very low frequencies, the signal level at these frequencies is reduced with consequent reduction in distortion due to wind noise and harmonics thereof.

Because of the low impedance level of the input circuit for the transducer at the frequencies of the carrier and its side-bands, the circuit is but poorly susceptible to noise due to leakage yacross the insulators in this circuit, the insulators including those in the microphone itself and others involved in this circuit.

Equalization for the lower frequencies is obtained without degradation of the signal-tonoise ratio.

Finally, the inherent rising characteristic of the microphone is compensated for over the entire range of frequencies to be translated and a uniform response isA attained throughout this range.

Although a specific transducer has been shown and described, it will be understood that it is but illustrative of this invention and that various modifications may be made therein without de- 7; parting from the; scope and spirit of .this inven-` tion asdefned in the appended claims.

What is Vclaimed is:

1. YAn electro-acoustic transducer comprising a condenser microphone having a diaphragm, a rst electrodeA opposite one face of said diaphragm andan auxiliary electrode opposite the other face of said diaphragm, a bridge, one arm of which includes said diaphragm and said first electrode and 'another arm of which includes a condenser substantially balancing the capacitance between said .diaphragm and said first electrode, means for applying a radio frequency polarizing voltage to said bridge such that the' output of said bridge is a radio frequency signal amplitude modulated in accordance with vibrations of said diaphragm, a radiov frequency amplifier energized in accordance with said signal, detector means coupled to the output Vof said amplifier, an audio frequency output circuit coupled to said detector, and a feedback coupling from said detector to said auxiliary electrode.

2. An electro-acoustic transducer in accordance with claim l wherein said microphone has a substantially cardioidal response pattern, said transducer comprising an equalizer network between said detector and said audio frequency output circuit for equalizing the response of the transducer for frequencies above a preassigned frequency in the range to be translated, and said feedback coupling including equalizer means for equalizing said response for frequencies below said Vpreassigned frequency.

3. An electro-acoustic transducer ,comprising a diaphragm and anV electrode adjacent thereto definingY a condenser, a bridge circuit, one arm of whichlisconstituted b y said condenser, said circuit including a condenser deiining an arm thereof adjacent said first condenser and substantially balancing the static capacitance of said rst condenser, means for applying a radio frequency polarizing Voltage across the two condensers in series, and connections to said circuit for deriving therefrom a signal of said Yradio frequency, amplitude modulated in accordance with vibrations'of said diaphragm.

4. An electro-acoustic transducer in accordance with claimY 3 comprising a radio frequency amplier energized in accordance with said radio freiquency amplitude modulated signal, anv auxiliary electrode adjacent said diaphragmand a negatiyefeedback coupling between said amplifier and said auxiliary electrode.

with said diaphragm and electrode defining therewith 'a directional microphone having a rising frequency response characteristic, and means energized in accordance with said amplitude modulated radio frequencysignal for applying to said Vdiaphragm a Vibration controlling force to compensate for said rising characteristic over a poragressie.

tion ofthe operating frequency range of the transducer j ft. ,An electro-acoustic transducer comprising a condenser microphone having `a directional response pattern and a risingV frequency response characteristic, said microphoneincluding a diaphragm and a main electrode adjacent thereto and defining a condenser'r therewith,vsaid microphone including also an auxiliary electrode opposte'saiddiaphragm, means for applying a radio frequency polarizing potential between sai'd diaphragm and said main electrode whereby the output o f said microphone is a radio frequency signal amplitude modulated in accordance with vibrations of said diaphragm,y an amplifier energized in accordance with the output of said microphone, means for detecting theoutput of said amplifier, and a negative feedback coupling between the output Ofsaid detector and said auxiliary electrode, said coupling comprising a network havinga loss characteristic which increases at substantiaily the same rate as the frequency response characteristic of said microphone, between preassigned frequencies in the lower portion of the range to be translated by said transducer.

'7. An electro-acoustic transducer in accordance with claim 6 comprising an audio frequency amplifier, and equalizer means between the output of said detector and said audio amplier for compensating for the rising microphone characteristic at frequencies above the higher of said preassigned frequencies.

8. An electro-acoustic transducer comprising a condenser Amicrop'l'ione including a diaphragm and an electrode in juxtaposition thereto, a bridge circuit .one armof which is constituted by said microphone, said-bridgelcircuit including acondenser balancing vthe static capacita-nce between said diaphragm and vsaid electrode. and defining an arm of #thev bridge `circuit adjacent said microphone and including alsoV a .center-tapped winding .connected across .said microphone and condenser and defining the `other two arms of said bridge circuit, ymeans lfor energizing said winding at radio frequency, and an output circuit connected' between .the center of said winding and the common terminal of said-.condenser'and microphone.

f REFERENCES CITED The following references are of record in the .le of this patent;

STATES PATENTS 

